The amount of costly cellulose fibers present in a paper substrate, in part, determines the density of the substrate. Therefore, large amounts of these costly cellulose fibers present in a paper substrate produce a more dense substrate at high cost, while low amounts of cellulose fibers present in a paper substrate produce a less dense substrate at a low cost. Reducing the density of a coated and/or uncoated paper product, board, and/or substrate inevitably leads to reduced production costs. This correlation remains true for all paper and substrate production and uses thereof, but is especially true, for example, in paper substrates used in envelopes, folding cartons, and in other packing applications. The substrates used in such envelope and packaging applications have a specified thickness or caliper.
By reducing the density of the paper substrate at a target caliper, less cellulose fibers are required to achieve the target caliper. In addition to a reduction in production costs, there is a production efficiency that is appreciated and realized when a paper substrate's density is reduced. This production efficiency is due, at least in part, to a reduction in drying requirements (e.g., time, labor, capital, etc.) of the paper substrate during production.
Examples of reducing density of the base paper substrate include the use of:                Multi-ply machines with bulky fibers, such as BCTMP and other mechanical fibers in the center plies of paperboard;        Extended nip press section for reducing densification during water removal; and        Alternative calendering technologies such as hot soft calendering, hot steel calendering, steam moisturization, shoe nip calendering, etc.However, these potential solutions involve high capital and costs. Thus, they may be economically infeasible.        
Further, even if the above-mentioned costly reduction in density methods are realized, thus producing a paper substrate having a target caliper, the substrate is only useful if such methodologies foster an acceptably smooth and compressible surface of the paper substrate. Presently, there are few potential low cost solutions to reduce the density of a paper substrate that has an acceptable smoothness and compressibility so that the substrate has a significant reduction in print mottle and an acceptable smoothness.
Low density coated and uncoated paper products, board, and/or substrates are highly desirable from an aesthetic and economic perspective. Unfortunately, current methodologies produce substrates that have poor print and/or printability quality. In addition, acceptable smoothness targets are difficult to attain using conventional methodologies.
One methodology to address the above-mentioned problems at lower cost is the utilization of expandable microspheres in paper substrates. These methodologies, in part, can be found in the following U.S. Pat. Nos. 6,846,529; 6,802,938; 5,856,389; and 5,342,649 and in the following U.S. Patent Publications: 2008/0017338; 2007/0044929; 2007/0208093; 2006/0000569; 2006/0102307; 2004/0065424; 2004/0052989; 2004/0249005 and 2001/0038893. The contents of each of these patents and publications are hereby expressly incorporated by reference in their entirety.
Many microspheres are found, when applied to the papermaking process, to have relatively low retention in the resultant paper substrate. As a result, the expandable microspheres are lost to the white water in the paper making process and the efficiency of the introduction of the expandable microspheres into the resultant paper substrate is low. U.S. Patent Publication Serial No. 2007/0044929 attempts to increase the retention of the microspheres by creating a composition having a much less negative charge than the base expandable microsphere compositions known previously.
Despite attempts to create a less dense and more bulky paper substrate, there remains a need in the art for a less costly and more efficient solution to reduce density and increase bulk while maintaining good performance characteristics such as smoothness and print mottle in the paper substrate.